Chapter 2: Disenfection By-Products

Disinfection By-Products

Drinking water needs to be disinfected to inactivate (or kill) pathogens. Chlorine has been the disinfectant of choice used by most potable water treatment plants. However, chlorine, chloramines, and chlorine dioxide disinfectants can react with naturally occurring matter in the water to form disinfection byproducts (DBP’s) including:

• Trihalomethanes (THM).
• Haloacetic acids (HAA).
• Chlorite.

Ozone disinfection can lead to the formation of a disinfection byproduct.

• Bromate.

The most common type of DBP’s are trihalomethanes (THMs).  Note that chlorine dioxide does not form THM’s but can react with bacteria to form other DBP’s. Many water agencies have switched from using the direct use of chlorine to chloramines to better manage the formation of DBP’s while also maintaining adequate disinfection residuals against harmful microbes.

Figure 2.1

The Information Collection Rule (ICR) was approved in May 1996 and required EPA to collect research data on:

• Disease-causing pathogens in drinking water sources (lakes, reservoirs, etc.), indicators of fecal contamination (Total coliform, coliform, fecal coliform E. coli).
• The amount of disinfectant (such as chlorine) and presence of DBP’s in treated drinking water.
• The effectiveness of certain treatment technologies.

This led to the adoption of Stage I and Stage II Disinfection and Disinfection Byproducts Rules to limit exposure to the DBP’s.

Disinfectants and Disinfection Byproducts Rules

The Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules (DBPRs) are part of the suite of Microbial and Disinfection Byproducts Rules (MDBPs). MDBPs are a series of interrelated regulations that address risks from microbial pathogens and disinfectants/disinfection byproducts.The Stage 1 Disinfectants and Disinfection Byproducts Rule (DBPR) reduces drinking water exposure to disinfection byproducts. The Rule applies to community water systems and non-transient non-community systems, including those serving fewer than 10,000 people that add a disinfectant to the drinking water during any part of the treatment process.The Stage 2 DBPR strengthens public health protection by tightening compliance monitoring requirements specifically for Trihalomethanes (TTHM) and Haloacetic acids (HAA5). The rule targets public water systems with the greatest risk.EPA has adopted enforceable regulations to limit occurrence of disinfection byproducts in drinking water for a group of four total trihalomethanes (TTHMs) (chloroform, bromodichloromethane (BDCM), dibromochloromethane (DBCM), and bromoform), a group of five haloacetic acids (HAA5) (monochloroacetic acid (MCA), dichloroacetic acid (DCA), trichloroacetic acid (TCA), monobromoacetic acid (MBA), and dibromoacetic acid (DBA)), and the individual byproducts chlorite and bromate. The maximum contaminant levels for these disinfection byproducts are:

• TTHMs (0.080 mg/L).
• HAA5 (0.060 mg/L).
• Chlorite (1.0 mg/L).
• Bromate (0.010 mg/L).

Taken together, the Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules (DBPRs) improve drinking water quality.Natural organic matter found in surface waters come about from the decay of plant, animal and microbial tissue. These compounds are collectively known as “Total Organic Carbon” (TOC) and are disinfection byproduct precursors. As such, treatment plants are required to monitor for their presence and remove a set percentage from incoming source water (usually through enhanced coagulation).Indeed, the Stage I and Stage II Disinfection Byproduct Rules set maximum residual disinfectant residual limits, DBP limits, requires DBP precursor removal, and establishes a rigorous monitoring plan.The maximum residual disinfection levels for the regulated disinfectants are:

• Chlorine – 4.0 mg/l (as Cl2).
• Chloramines – 4.0 mg/l as (Cl2).
• Chlorine Dioxide – 0.8 mg/l as (ClO2).

The rules also differentiate on whether the source is surface water, groundwater, or groundwater under the direct influence of surface water.EPA list the factors affecting Total THM and HAA5 formation as:

• Type of disinfectant.
• Disinfectant dosage.
• Disinfectant contact time.
• Type and level of DBP precursor.
• Disinfection point.
• pH and water temperature.
• Residence time in the distribution system.

Disinfectants other than chlorine compounds to manage DBP’s.

Chemical Disinfection

Iodine

Iodine is a good disinfectant and has been utilized for potable water treatment since the early 1900’s in concentrations between 2.5 – 7 ppm. The US Army continues to issue iodine-based tablets to field soldiers and other personnel to disinfect water in emergencies.  It is expensive and is also not recommended for use as a primary disinfectant due to lack of knowledge on long-term toxic effects.

Bromine

Bromine is primarily used as an alternative disinfectant for swimming pools, spas, and cooling tower water, but not for municipal drinking-water, partly due to cost and partly to concerns about the formation of brominated DBPs.

Ozone

Ozone is used to disinfect water along with reducing taste and odors. Drawbacks of ozone for disinfection include a high cost, big physical footprint, lack of residual, difficulty in storing, and maintenance requirements.

Ozone has its advantages and disadvantages. Ozone treatment has the ability to achieve higher levels of disinfection than chlorine or UV, however, the capital costs as well as maintenance expenditures are not competitive with available alternatives.

Ultraviolet Rays

Advances in UV disinfection have been made in recent years. An Ultraviolet (UV) disinfection system transfers electromagnetic energy from a mercury arc lamp to an organism’s genetic material (DNA and RNA). When UV radiation penetrates the cell wall of an organism, it destroys the cell’s ability to reproduce. UV radiation, generated by an electrical discharge through mercury vapor, penetrates the genetic material of microorganisms and retards their ability to reproduce. Unfortunately, this method leaves no disinfectant residual and is expensive.

Heat

Primarily used in emergencies, heating water to a boil for 5 minutes will kill all microorganisms which may have breached the distribution system. From a practical perspective, it is an expensive and inefficient way to disinfect water for a public water system.

UV Disinfection and Treatment of Water

Ultraviolet (UV) rays are part of the light that comes from the sun. The UV spectrum is higher in frequency than visible light and lower in frequency compared to x-rays. The UV spectrum has a larger wavelength than x-rays and a smaller wavelength than visible light and the order of energy, from low to high, is visible light, UV, and x-rays.

UV is known to be an effective disinfectant due to its strong germicidal (inactivating) ability. UV disinfects water containing bacteria and viruses and can be effective against protozoans, such as Giardia lamblia cysts or Cryptosporidium oocysts. UV is used in the pharmaceutical, cosmetic, beverage, and electronics industries. In the United States, it is used for drinking water disinfection; however, high operating costs compared to disinfection by chlorination has limited its usage.

Because of safety issues associated with the reliance of chlorination and improvements in UV technology, UV has experienced increased acceptance in municipal water systems. Two classes of disinfection systems are certified and classified by the NSF under Standard 55, Class A and Class B Units.

1. Class A These ultraviolet water treatment systems must have an intensity and saturation rating of at least 40,000 uw-sec/cm2 and possess designs that will allow them to disinfect and/or remove microorganisms from contaminated water. Affected contaminants should include bacteria and viruses. Class A point-of-entry and point-of-use systems covered by this standard are designed to inactivate and/or remove microorganisms, including bacteria, viruses, and Cryptosporidium oocyst and Giardia cysts from contaminated water. Systems covered by this standard are not intended for the treatment of water that has obvious contamination or intentional source contamination, such as raw sewage, nor are these systems intended to convert wastewater to drinking water. These systems are intended to be installed on visually clear water.
2. Class B These ultraviolet water treatment systems must have an intensity and saturation rating of at least 16,000 uw-sec/cm2 and possess designs that will allow them to provide supplemental bactericidal treatment of water already deemed safe, such that no elevated levels of E. coli or a standard plate count of less than 500 colonies per 1 ml exists. NSF Standard 55 suggests Class B UV systems are designed to operate at a minimum dosage and are intended to reduce normally occurring non-pathogenic or nuisance microorganisms only. The Class B or similar non-rated UV systems are not intended for the disinfection of microbiologically unsafe water.

The type of unit depends on the situation for use, source of water, and water quality. Transmitted UV light dosage is affected by water clarity. Water treatment devices are dependent on the quality of the raw water. When turbidity is 5 NTU or greater and/or total suspended solids are greater than 10 ppm, pre-filtration of the water is highly recommended. Normally, it is advisable to install a 5-to-20-micron filter prior to a UV disinfection system.

UV disinfection is based on the principles associated with wave lengths of light that damage the nucleic acids of water borne pathogens. UV radiation has three wavelength zones, UV-A, UV-B, and UV-C, and it is the last region, the shortwave UV-C, which has germicidal properties for disinfection. A low-pressure mercury arc lamp resembling a fluorescent lamp produces the UV light in the range of 254 manometers (nm). These lamps contain elemental mercury and an inert gas, such as argon, in a UV-transmitting tube, usually quartz. Traditionally, most mercury arc UV lamps have been the low-pressure type because they operate at a relatively low partial pressure of mercury, low overall vapor pressure (about 2 mbar), low external temperature (50-100o C), and low power. These lamps emit nearly monochromatic UV radiation at a wavelength of 254 nm, which is in the optimum range for UV energy absorption by nucleic acids (about 240-280 nm).

In recent years, medium pressure UV lamps that operate at much higher pressures, temperatures, and power levels have been installed. They emit a broad spectrum of higher UV energy between 200 and 320 nm.

Figure 2.2: UV Disinfection Unit

An essential requirement for UV disinfection with lamp systems is an available and reliable source of electricity. While the power requirements of low-pressure mercury UV lamp disinfection systems are modest, they are essential for lamp operation to disinfect water. Since most microorganisms are affected by radiation around 260 nm, UV radiation is in the appropriate range for germicidal activity. UV lamps are available that produce radiation in the range of 185 nm, and they are effective in reducing microorganisms as well. They will also reduce the total organic carbon (TOC) content of the water.

For typical UV systems, approximately 95-percent of the radiation passes through a quartz glass sleeve and into the untreated water. The water is flowing as a thin film over the lamp. The glass sleeve is designed to keep the lamp at an ideal temperature of approximately 104° F.

UV radiation affects microorganisms by altering the DNA in the cells and impeding reproduction. UV treatment does not remove organisms from the water. It inactivates them. The effectiveness of this process is related to exposure time and lamp intensity, as well as general water quality parameters.

The exposure time is reported as microwatt-seconds per square centimeter (μwatt-sec/cm2), and the U.S. Department of Health and Human Services has established a minimum exposure of 16,000 µwatt-sec/cm2 for UV disinfection systems. Most manufacturers provide a lamp intensity of 30,000-50,000µwatt-sec/cm2. In general, coliform bacteria are destroyed at 7,000 µwatt-sec/cm2.

Since lamp intensity decreases over time with use, lamp replacement and proper pretreatment are key to the success of UV disinfection. In addition, UV systems should be equipped with a warning device to alert operators when lamp intensity falls below the germicidal range.

Used alone, UV radiation does not improve the taste, odor, or clarity of water. UV light is a very effective disinfectant, although the disinfection can only occur inside the unit. No residual disinfection in the water exists to inactivate bacteria that may survive or may be introduced after the water passes by the light source. The percentage of microorganisms destroyed depends on the intensity of the UV light, the contact time, raw water quality, and proper maintenance of the equipment.

If material builds up on the glass sleeve or the particle load is high, the light intensity and the effectiveness of treatment are reduced. At sufficiently high doses, all waterborne enteric pathogens are inactivated by UV radiation. The general order of microbial resistance (from least to most) and corresponding UV doses for extensive (>99.9%) inactivation are vegetative bacteria and the protozoan parasites Cryptosporidium parvum and Giardia lamblia at low doses (1-10 mJ/cm2) and enteric viruses and bacterial spores at high doses (30-150 mJ/cm2).

Figure 2.3: UV light used for disinfection in water treatment.

Most low-pressure mercury lamp UV disinfection systems can readily achieve UV radiation doses of 50-150 mJ/cm2 in high quality water; and therefore, efficiently disinfect waterborne pathogens. However, dissolved organic matter, such as natural organic matter, certain inorganic solutes such as iron, sulfites and nitrites, and suspended matter (particulates or turbidity) will absorb UV radiation or shield microbes from UV radiation, resulting in lower delivered UV doses and reduced microbial disinfection. Another concern surrounding disinfecting microbes with lower doses of UV radiation is the ability of bacteria and other cellular microbes to repair UV-induced damage and restore pathogenicity, which is a phenomenon known as reactivation.

UV inactivates microbes primarily by chemically altering nucleic acids. However, the UV-induced chemical lesions can be repaired by cellular enzymatic mechanisms, some enzymes act independent of light (dark repair) and other enzymes require visible light (photo-repair or photo-reactivation). Therefore, achieving optimum UV disinfection of water requires delivering a sufficient UV dose to induce greater levels of nucleic acid damage; and thereby, overcome DNA repair mechanisms.

UV units have a maximum flowrate capacity and some equipment have minimum flowrates. If the flow is too high, water will pass through without enough UV exposure. If the flow is too low, heat may build up which can damage the UV lamp. A UV unit with minimum flow requirements should not be placed on the water line supplying pressure stations in a non-recirculating system. UV units are most often used in constant flow systems.

UV lamps do not burn out as normal florescent lamps do. Instead, the UV lamps will solarize, which reduce their intensity to about 60% of a new lamp after about one year of continuous use. When lamps are new, they will generate a dosage level near 60,000 µW-s/cm2. When the dosage drops to 30,000 µW-s/cm2, the minimum dosage needed to effectively kill bacteria, lamps should be replaced. Lamp life will be shortened significantly if the lamp is turned on and off more frequently than once every eight hours.

Water should be sampled and tested for bacteria counts regularly. Sample before and after the UV unit to test its performance. Water should also be sampled in the distribution system since bacterial regrowth can occur downstream of the UV unit.

As water passes through the UV unit, minerals, debris, and other material in the water will deposit onto the quartz or Teflon sleeve. This activity will limit the penetration of UV rays through the sleeve and into the water. To maintain high clarity, the glass around the lamp must be cleaned regularly. Cleaning frequency depends on the water quality and will be minimal with RO treatment upstream.

UV light intensity meters are available which indicate the penetration of UV light through the glass sleeve and the water. Low intensity means the UV dose is too low to provide adequate disinfection. This meter will indicate when cleaning or lamp replacement is needed.

Ozone

Ozone is one of the most powerful water treatment compounds available to system managers today. It is a technology that has been in continual commercial use for over 100 years and has distinct properties that allow disinfection of even heavily compromised water streams. With the 1996 reauthorization of the Safe Drinking Water Act, ozone was named as among the best available technologies for water system compliance with National Primary Drinking Water Regulations as overseen by the US Environmental Protection Agency.

Ozone (O3) is formed when oxygen molecules are exposed to electron flow. Ozone molecules are unstable and will lose the third oxygen atom over time. Ozone is formation characterization:

1. Ozone generators provide an electron flow between dielectric and SS tubes.
2. Oxygen is passed through the gap between dielectrics resulting in ozone generation.
3. Oxygen feed gas must be dry and free of particles.
4. Ozone generators must be cooled, and cooling water removes .90 percent of the heat that is generated.

Ozone is a powerful oxidant with high disinfectant capacity. Ozone residuals between 0.3 to 2.0 mg/L inactivate viruses. Inactivation rates range from >3.9-log to >6-log, and occur within very short contact periods, 5 seconds. Microorganisms in natural waters are very sensitive to ozone. Giardia and enteric viruses are inactivated by ozone, as a primary disinfectant, with 5 minutes of contact time. Ozone residuals of 0.5 to 0.6 mg/L result in 3-log and 4-log removals, respectively. When ozone is used as a primary treatment, the criterion for its use is based on ozone residuals, competing ozone demands, and a minimum contact time to meet the required cyst and viral inactivation requirements.

Ozone is the strongest oxidant and strongest disinfectant available for potable water treatment. This unique material can be utilized for a number of specific water treatment applications, including disinfection, taste and odor control, color removal, iron and manganese oxidation, hydrogen sulfide removal, nitrite and cyanide destruction, oxidation of organics such as phenols, pesticides, and some detergents, algae destruction, and removal, and as a coagulant aid. Even though ozone is the strongest chemical disinfectant available for water treatment, some refractory organics are not oxidized, or oxidize too slowly. In such cases, ozone can be combined with UV radiation and/or hydrogen peroxide to produce hydroxyl free radicals, HO-, which is a stronger oxidant than molecular ozone, O3. Deliberate production of hydroxyl free radicals starting with ozone has been termed ozone advanced oxidation. Groundwater that is contaminated with chlorinated organic solvents and some refractory hydrocarbons are being treated successfully with ozone advanced oxidation techniques.

At ambient temperatures, ozone is an unstable gas, partially soluble in water; generally, more soluble than oxygen. Due to its instability, ozone quickly reverts to oxygen. Ozone cannot be produced at a central manufacturing site, bottled, shipped, and stored prior to use. It must be generated and applied on-site. The installation of an ozone production plant requires storage of pure oxygen on-site as the feed gas. Ozone is generated for commercial uses using corona discharge or ultraviolet radiation. The UV technique produces low concentrations of ozone, whereas corona discharge produces ozone concentrations in the range of 1 – 4.5 % when dry air is fed to the ozone generator. When concentrated oxygen is used as the feed gas, gas phase ozone concentrations of up to 14 to 18% can be produced. Since ozone is only partially soluble in water, once it has been generated it must be contacted with the water to be treated in such a manner as to maximize the transfer of ozone from the gas phase into water. For this purpose, many types of ozone contactors have been developed. However, as higher concentrations of ozone gas are employed, contacting system designs become more critical because of the lower gas to liquid ratios.

The use of oxygen as the feed gas can result in oxygen super saturation of the treated water causing operational problems and corrosion in the distribution system. Ozone contacting system options include atmospheric tall towers or pressurized gas to liquid mass transfer processes. Fine bubble diffusers, static mixers, or venturi injectors can be used to mix the gas with the water to be treated in full flow or side stream configurations. Once dissolved in water, ozone is available to act on water contaminants to accomplish its intended purposes of disinfection and/or oxidation. At pH levels of 3-6, ozone is present primarily in its molecular form (O3). However, as the pH rises, the decomposition of ozone to produce the hydroxyl free radical (HO-) becomes increasingly rapid. At pH 7 about 50% of the ozone transferred into water produces HO-. At pH >10, the conversion of molecular O3 to HO- is virtually instantaneous.

Because ozone is such a powerful oxidant/disinfectant, the trick to applying it to solve water treatment problems is to do so in a manner that is effective for water treatment, yet at the same safe for the people in the vicinity. Ozone safety issues are handled easily by using proper ambient ozone monitoring, tank venting, and ozone destruction. In the case of systems driven solely by a pumping/injector system, ozone may be produced under vacuum, which ensures no leakage of ozone into the operating environment.

The five basic components of an ozone system include:

1. Gas preparation–either drying gas to a suitable dew point or using oxygen concentrators.
2. A suitable electrical power supply.
3. A properly sized ozone generator(s).
4. An ozone contacting system.
5. Ozone off-gas destruction or suitable venting system.

Moisture in the feed gas causes two operating problems:

1. The amount of ozone produced by application of a given electrical energy level is lowered as relative humidity rises. Consequently, it is usually cost-effective to dry the air to a recommended dew point of minus 65’C (-65’C or -76’F) or lower.
2. Ozone generated using air in the presence of moisture allows small amounts of nitrogen oxides to react with the moisture to produce nitric acid. In this instance, gas condensation at the cooling/heat transfer surfaces produces a corrosive compound which can cause corrosion problems in the ozone generation equipment with concomitant increases in equipment maintenance requirements.

Because of the high oxidative qualities of gas-phase ozone and the chance of moisture from a failing feed gas unit, system managers must take extra care to make certain that all components in the ozone generator, ozone supply line, ozone gas to liquid mass transfer equipment and the contact vessel are ozone-compatible.

For large scale ozone systems, the equipment for cleaning and drying feed gases can become quite complex. For example, effective air drying can involve multiple treatment steps including air filtration, compression, cooling, desiccation, and final filtration prior to passage into an operating corona discharge ozone generator.

A need exists for efficient ozone contacting and destruction of excess ozone in contactor off-gases. Absent an effective ozone off-gas destruct unit, excess ozone would be present for people in the vicinity to breathe, which is not recommended because of its strong oxidizing nature. Additionally, ozone is heavier than ambient air, and can settle in the vicinity, and attack oxidizable materials. Destruction of contactor off-gas ozone is readily accomplished thermally (370’C), catalytically, thermal-catalytically, and by passing the off-gas through granular activated carbon. Care should be exercised in selecting an ozone destruct method whenever very high concentrations of ozone are encountered.

Ozone is a critical process for non-reverse osmosis purification. It is usually coupled with biologic activated carbon filtration. The process reduces TOC and trace chemical pollutants, removes protozoans, kills viruses, and is a flocculation aid. Ozone treatment is an oxidation process used as a disinfection and oxidant prior to biologic activated carbon filtration.

Instrumentation and controls for ensuring effective and safe operation of ozone systems are concerned with applying ozone effectively and affordably. System processes control ozone generation, oxygen usage, drying, ozone injection and diffusing, and ozone destruction. The instrumentation monitors each step, and each step has an alarm associated with the process.

Review Questions

1. Describe disinfection by-products.
2. Explain the Disinfection By-Products Rule.
3. Describe the formation of disinfection by-products.

Test Questions

1. What is the MCL for TTHMs?

1. 0.080 milligram per liter.
2. 0.10 milligrams per liter.
3. 80 milligrams per liter.
4. 60 micrograms per liter.

1. What is used to determine compliance with the maximum contaminant levels for two groups of DBPs in the Stage II Disinfection By-products Rule (TTHMs and HAA5) in the distribution system?

1. random sites.
2. each monitoring location.
3. dead ends.
4. the first tap.

1. Given higher temperature of water, how does efficiency of a disinfectant change?

1. decreases.
2. no change.
3. increases.
4. none are correct (more factors influence the efficiency and temperature efficiency cannot be determined).

1. Which is an example of undesirable compounds formed by reactions between chlorine and organics?

1. benzene.
2. chlorite.
3. trihalomethanes (THMs and HAA5s).
4. chlorate.

1. Which does not form THM’s that may be formed by other chlorine compounds?

1. gas chlorine.
2. sodium hypochlorite.
3. calcium hypochlorite.
4. chlorine dioxide.

1. Which can be used to destroy pathogenic microorganisms?

1. X-rays.
2. radium.
3. ultraviolet light.
4. radon.

1. Which lacks the ability to leave a residual in the distribution system?

1. ozone.
2. chloramines.
3. chlorine dioxide.
4. calcium hypochlorite.

1. Which has proven effective in reducing the formation of THMs and other DBPs and maintaining a detectable residual throughout the distribution system?

1. UV light.
2. ozone.
3. heat.
4. chloramines.

1. Which can be used as a disinfectant and does not form carcinogenic compounds that may be formed by disinfectants and is not affected by ammonia, and it is a very effective disinfectant at higher pH levels?

1. UV light.
2. ozone.
3. chlorine dioxide.
4. sodium hypochlorite.

1. Which can be used after filtration for bacterial disinfection and viral inactivation or before coagulation for treating iron and manganese, helping flocculation, and removing algae?

1. UV light.
2. ozone.
3. chlorine dioxide.
4. chloramines.

References

• Disinfection By-Products (DBPs) by USDHHS
• Stage 1 and Stage 2 Disinfectants and Disinfection By-Products Rules by USEPA
• Disinfectants and Disinfection By-Products Rules (DBPRs) (Stage 1 & 2) by USEPA
• Water Systems, Disinfection By-Products, and the Use of Monochloramine by USEPA
• World Health Organization, Bromine as a drinking-water disinfectant, 2018, ISBN 978-92-4-151369-2
• EPA Wastewater Technology Fact Sheet Ozone Disinfection EPA 832-F-99-063, September 1999
• EPA Wastewater Technology Fact Sheet Ultraviolet Disinfection EPA 832-F-99-064, September 1999

Images